Performance enhancement of humidity sensors made from oxide heterostructure nanorods via microstructural modifications

Abstract

We reported the fabrication of a highly sensitive and fast switchable humidity sensor based on ZnO–TiO₂ core–shell nanorods that were synthesized using hydrothermal solution and atomic layer deposition. These nanorods were thermally treated under various physical conditions to improve their sensing performance. The structural investigation revealed that the crystal and microstructure changed with the thermal treatment. Notably, the amorphous TiO₂ shell layer transformed into various degrees of crystalline phase after annealing in air and a vacuum at 400 °C. Furthermore, the responses of the sensors fabricated from the ZnO–TiO₂ nanorods, with and without thermal annealing, to relative humidity (RH) changes were proportional with the increase in humidity. Among various samples, ZnO–TiO₂ nanorods thermally treated in a vacuum exhibited the highest humidity selectivity in response to the cyclic changes in humidity from 11% RH to 33–95% RH at room temperature. Possible mechanisms for the enhancement of sensor performance have been discussed based on structural modifications caused by the thermal treatments.